HVDC is very cost efficient to transmit bulk power from distant power plants to consumers. One drawback has been the difficulty to connect loads along the line. With an ac-transmission system such connection of a load along the line may be brought about by the use of a transformer. For an HVDC transmission system such connection along the line requires a converter station having the same capacity as the receiving station. This capacity requirement would apply also for a small tap of power. Furthermore, disturbances in the ac net at the tap might, at least temporarily, interrupt the main HVDC transmission. This has in some cases led to a HVDC transmission alternative being excluded.
From U.S. Pat. No. 5,187,651 (Ekström) a system for discharging electrical power from a HVDC line to a local network is previously known. The system comprises a first converter connected in series with the HVDC line. The first converter and a second converter constitute an ac network containing a first transformer. The second converter and a third converter constitutes a dc network for feeding a local ac network via a second transformer.
From WO 9515605 (Asplund) a device for tapping electric power from a HVDC transmission line is previously known. The known system comprises a first converter connected in series with the HVDC transmission line. The first converter and a second converter located on ground potential constitute a high frequency ac network containing a first and second transformer and capacitor means for potential-separation. The second converter and a third converter constitutes a dc network for providing an ac current to a local network.